SthK-bPAC is an engineered light-gated K[+] channel. It can be used as an optogenetic tool to inhibit neuron activity; it is activated by blue light. SthK-bPAC is a fusion protein composed of the cyclic nucleotide-gated K[+] channel encoded by the Spirochaeta thermophila SthK gene (UniProtKB:E0RR11), a plasma membrane trafficking signal (KSRITSEGEYIPLDQIDINV), the Venus fluorescent protein, the Beggiatoa BLUF domain regulated adenylyl cyclase (bPAC) and an endoplasmic reticulum export signal (FCYENEV) (FBrf0240386).